Filter compensation for switching amplifiers

ABSTRACT

Embodiments of the present invention provide methods and systems for limiting bipolar current flow in a switching amplifier. Embodiments of the present invention are directed to a multi-referenced switching amplifier. In some embodiment, the switching amplifier is a dual referenced switching amplifier comprising a regulator between the two references, where the first reference provides coarse modulation to a load and the second reference provides fine modulation to the load. The dual referenced switching amplifiers comprise the output filters made up of an inductor and a capacitor. In some embodiments, fine modulation is not applied to the load, thus limiting bipolar current flow induced by the filter inductors.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of pending U.S. patent applicationSer. No. 12/524,841, filed Jul. 28, 2009, which application claimspriority to International Application No. PCT/US2008/052533 filed Jan.30, 2008, which claims priority to U.S. Provisional Application No.60/887,173, filed Jan. 30, 2007. The entire disclosure of these priorapplications are considered to be part of the disclosure of the instantapplication and are hereby incorporated by reference therein.

TECHNICAL FIELD

This invention is directed to amplifiers, and more particularly toswitching amplifiers.

BACKGROUND OF THE INVENTION

Switching amplifiers enjoy significantly better efficiency than theirnon-switching predecessors. However, significant improvements arecontinuously being made to switching amplifiers to improve fidelity. Onerecent improvement is the use of multiple references which areselectively switched to a load, often called multi-reference switchingamplifiers. Such a circuit configuration is described in U.S. Pat. No.6,535,058 Multi-reference, High-Accuracy Switching Amplifier, the entirecontent of which is incorporated herein by reference. Multi-referenceswitching amplifiers typically comprise an additional regulator thanconventional class D switching amplifiers.

A current problem with switching amplifiers involves radio frequency(RF) interference. In order to minimize (RF) interference to surroundingreceptors passive output filters using inductors and capacitors arecommonly employed after the output stages of multi-reference switchingamplifiers. The inductance presented by these filters requires that theregulator control bipolar current at unexpected voltages. This bipolarcurrent requirement is induced by the inductance of the output filters,which places large demands on the regulators used in multi-referenceamplifiers, potentially negating the efficiency benefits normallyexpected from switching amplifiers. In addition, regulators that arecapable of both sinking and sourcing current are much more expensive. Itwould be preferred to use less expensive regulators, which are capableof allowing current to flow in one direction rather than two.

Therefore, a need exists for a method and system where referenceregulators of multi-reference switching amplifiers are protected fromthe bipolar current flow induced by filter inductance.

SUMMARY OF THE INVENTION

The present invention is directed to a system and method of limitingbipolar current flow in a switching amplifier. In one aspect of theinvention, a switching amplifier comprising a first voltage source and asecond voltage source, the second voltage source being supplied by aregulator. In addition, a first set of switching devices coupled to thefirst voltage source, and a second set of switching devices coupled tothe second voltage source. A controller is configured to control theswitching devices to provide modulated pulsewidths to a load, thecontroller preventing at least one of the second set of switchingdevices from being coupled to the load when coupling to the load wouldbe expected to cause the regulator to source current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a multi-reference switching amplifier usinga single regulated secondary reference level according to one embodimentof the invention.

FIG. 2 shows the output voltage and current waveforms typically seen inthe amplifier of FIG. 1 in accordance with prior art.

FIG. 3 shows the output voltage and current waveforms of the amplifierof FIG. 1 according to one embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the present invention are directed toward providing asystem and method of limiting bipolar current flow in a switchingamplifier. Certain details are set forth below to provide a sufficientunderstanding of the invention. However, it will be clear to one skilledin the art that the invention may be practiced without these particulardetails.

FIG. 1 is a block diagram of a dual reference switching amplifieraccording to one embodiment of the invention. Although FIG. 1 isdirected to a dual reference switching amplifier, a person of ordinaryskill in the art would appreciate that the invention applies to anymulti-reference switching amplifier. The dual reference switchingamplifier of FIG. 1 has two references, positive power supply voltage V+and the voltage supplied from V+ by a regulator 114. Positive powersupply voltage V+ supplies energy to a load 119 through control switches108, 111. The regulator 114 supplies energy to the load 119 throughswitches 109, 112. Ground is provided to the load 119 through switchingdevices 110, 113. In some embodiments, the voltage supplied by the V+reference is significantly greater than the voltage supplied by theregulator 114. For instance, in one embodiment, the voltage supplied bythe positive power supply voltage V+ reference is approximately 12V, andthe voltage supplied by the regulator 114 is 47 mV.

Input datastream 100 is applied as input to pulsewidth modulator (PWM)107, which outputs PWM control signals 101, 102, 103, 104, 105, and 106to control switching devices 108, 109, 110, 111, 112, and 113,respectively. The load 119 is connected in a bridge configuration acrosstwo independent output stages. A first output stage A is throughswitching devices 108, 109, and 110. A second output stage B is throughswitching devices 111, 112, and 113. The first output stage A is coupledto an inductor 115, which is further coupled to a capacitor 116. Theinductor 115, in conjunction with the capacitor 116, filters the outputsof switching devices 108, 109, and 110 before applying the outputs to afirst terminal of the load 119. The second output stage B is coupled toan inductor 117, which is further coupled to a capacitor 118. Theinductor 117, in conjunction with the capacitor 118, filters the outputsof switching devices 111, 112, and 113 before applying the outputs to asecond terminal of the load 119. Therefore, switching devices 108 and111 provide gate connection of inductors 115 and 117, respectively, tothe positive power supply V+, switching devices 109 and 112 provide gateconnection of inductors 115 and 117, respectively, to the referencevoltage supplied from V+ by regulator 114, and switching devices 110 and113 provide gate connection of inductors 115 and 117, respectively, toground. The circuit of FIG. 1 is described in further detail in U.S.Pat. No. 6,535,058 referenced above.

The circuit of FIG. 1 results in two separate PWM datastreams on eachside of the load 119. In one embodiment, the PWM datastream that isapplied to the load 119 through switching devices 108, 111 is a coarsehigh voltage modulated stream. The pulse width modulated datastream thatis applied to the load 119 through switching devices 109, 112 is a finelow voltage modulated stream. However, as will be apparent to a personof ordinary skill in the art, the configuration of the coarse and finevoltage stream is dependent upon which switching devices are coupled tothe high voltage source and the low voltage source.

FIG. 2 shows the output voltage and current waveforms typically seen inthe amplifier of FIG. 1 in accordance with prior art. Voltage traces201, 202, 203, 204, 205, and 206 reflect the states of control signals101, 102, 103, 104, 105, and 106 of FIG. 1, respectively, as the outputpolarity to the load 119 in FIG. 1 changes. Voltage trace 207 shows thecollective outputs of switching devices 108, 109, and 110, as input toinductor 115 at first input stage A. Voltage trace 208 shows thecollective outputs of switching devices 111, 112, 113, as input toinductor 117 at second input stage B. As is commonly done with switchingamplifiers, the modulation of the datastreams through switching devices108, 111, which can be seen in voltage traces 201 and 204, and themodulation of the datastreams through switching devices 109, 112, whichcan be seen in voltage traces 202 and 205, are additive to a minimumpulsewidth. In order to avoid offsets imposed by unbalanced pulsewidths,equal minimum pulsewidths are typically provided on both sides of theoutput bridge. Thus, often requiring the regulator to both source andsink current.

Current trace 209 indicates signed current in the load 119. Currenttrace 210 indicates resultant signed output current required of theregulator 114. Current trace 209 indicates signed current in the load119, with the positive y-axis movement indicating current sinking on theA side of the load 119. Current trace 210 indicates resultant signedoutput current required of the regulator 114, with the positive y-axisindicating current sinking. In reference to time marker 212, coarsemodulated pulsewidths are no longer being applied to the A side of theload 119, and fine modulated pulsewidths are no longer being applied tothe B side of the load. However, a reference voltage minimum pulsewidthare applied to the A side of the load. The minimum pulsewidth amechanism whereby the effects of charge injection of output switchingdevices is mitigated as taught in U.S. Pat. No. 6,937,090, which isincorporated herein by reference. Also note, V+ minimum pulsewidth isapplied to the A side of the load just before timemarker 213.

In particular, the pulses in trace 210 indicate whether the regulator issourcing or sinking current. Note that current trace 210 shows unipolarcurrent until time marker 211. During this time the regulator 114 issinking current as indicated by the positive pulses. For instance,before time marker 211, at the first output stage A, the voltage beingapplied to the inductor 115 is V+, and at the second output stage B, thevoltage being applied to the load is also V+. This does not produce acurrent in the load 119. A moment later, the voltage at the secondoutput stage B reduces from V+ to the regulated reference voltage whilethe first output stage A remains at the V+ voltage. At that instance,the inductor 117 is expecting a large voltage, but receives a lowervoltage. The load 119 has a high current, and the inductor 117 pullscurrent from the load 119, thus, causing the regulator 114 to sinkcurrent.

At time marker 211, the activation of switching device 112 followed bythe activation of switching device 109, causes current reversal at theoutput of regulator 114. As stated above, just before marker 211 a V+voltage is being applied to the first output stage A and no voltage isbeing applied to the second output stage B. At the moment of marker 211,the voltage applied to the first output stage A goes from a V+ voltageto the reference voltage and the voltage applied to the second outputstage B is ground. However, the inductor 115 is expecting a V+ voltage.Because there is high current in the load 119, the inductor 117 pullscurrent from the load 119 and tries to pull more current from switchingdevice 109. This results in the regulator 114 sourcing current. The samephenomenon occurs at time marker 212, with a larger current changeresultant of a higher output current in the load 119. As can be seen,large unpredictable current swings through zero must be supported by thereference regulator, which requires a more expensive regulator andtypically results in distortion.

Turning now to time marker 213. Time marker 213 does not cause theregulator 114 to source current. Just before time marker 213, a V+voltage is applied to the first output stage A, and the referencevoltage is applied to the second output stage B. At time marker 213 thevoltage at the input of the inductor 115 reduces from V+ to thereference voltage, and the voltage at the input of the inductor 117remains at the reference voltage. This does not require the regulator tosource current to the load 119. Rather, it forms a shunt. In particular,the reference voltage on each side of the load 119 creates a shortcircuit across the inductors 115 and 117, and inductors 115 and 117 areable to pull current from each other, rather than source current fromthe regulator 114.

FIG. 3 shows the output voltage and current waveforms of the switchingamplifier of FIG. 1 according to one embodiment of the invention.Voltage traces 301, 302, 303, 304, 305, and 306 reflect the states ofcontrol signals 101, 102, 103, 104, 105, and 106 of FIG. 1,respectively, as the output polarity to the load 119 changes in the samefashion as in FIG. 2. Voltage traces 307 and 308 indicate resultantinputs to inductors 115 and 117, respectively. Current trace 309indicates signed current in the load 119. Current trace 310 indicatesresultant signed output current required of the regulator 114.

In comparison with FIG. 2, note that current trace 310 of FIG. 3 showsunipolar current at both timemarkers 311 and 312, thus indicating thatthe regulator is only sinking current and not sourcing current. Inparticular, note the absence of minimum pulsewidths in trace 302 whenthe pulsewidth of trace 301 exceeds that of trace 304. The widepulsewidth of trace 301 creates a positive voltage at the first outputstage A. In particular, the modulator 107 applies coarse modulation tothe load 119 through switching device 108 and does not apply minimumpulsewidths to the load 119 through switching device 109. However, onthe other side of the bridge, the modulator 107 applies minimum pulsewidths through switching device 111 and fine modulation to the load 119through switching device 112. Not applying minimum pulsewidths to thefirst output stage A prevents the regulator 114 from having to sourcecurrent to the load 119.

Similarly, note the absence of minimum pulsewidths in trace 305 when thepulsewidth of trace 304 exceeds that of trace 301. Although, it is notnecessary to prevent the secondary reference pulsewidths in thissituation as discussed above in reference to FIG. 2 and time marker 213,it may be done to simplify the algorithm in the controller whichproduces the pulsewidths, such as in the modulator 107.

In summary, the absence of minimum pulsewidth in traces 302 and 305prevents the regulator 114 from having to source current. Although themethod described above comprises a regulator that is operable to sinkcurrent rather than source current, it will be apparent to a person ofordinary skill in the art that the regulator may be operable to sourcecurrent rather than sink current based on the voltage configuration ofthe switching amplifier. Therefore, the regulator 114 is generally thetype that is operable to allow high current to flow in one direction andnot in the other.

As will be apparent to a person of ordinary skill in the art, thevoltage configuration in FIG. 1 may be of another configuration. Forinstance, in one embodiment, the V+ power supply may be ground and theground shown in FIG. 1 may be a negative voltage. This will require aregulator 114 that is operable to sink current but not operable tosource current as the embodiment disclosed in FIG. 1. Anotherembodiment, however, may include V+ in FIG. 1 as a negative voltage.This would result in the regulator 114 being operable to source currentrather than sink current. Similarly, if V+ was ground and the currentground shown in FIG. 1 is a positive voltage, that would also result ina regulator that is operable to source current and not sink current.Therefore, the regulator 114 is generally the type that is operable toallow current to follow in one direction and not in the other.Therefore, the regulator 114 may be operable to sink or source currentdepending on the configuration of the voltage in the switchingamplifier.

As will be apparent to a person of ordinary skill in the art, themodulator 107 determines whether to apply the reference pulsewidth toprevent the regulator 114 from sourcing current. However, an upstreamprocessor or preprocessor may be used to determine whether to apply thereference pulsewidth to prevent the regulator 114 from sourcing currentbased on the history of the audio input signal. In particular, anupstream controller operable to predict inductor current, furtherreducing distortion.

Although secondary to the content of the present invention, the outputoffsets induced by the removal of the reference voltage through theswitching devices 109 and 112 are assumed to be removed by compensatoryoffsets in the modulator. Therefore, even though the removal of thesecondary pulsewidth result in a voltage imbalance across the load, itreduces distortion and removes large strains placed on the regulator114. Thus, it can be seen that sign-gated application of secondaryreference pulsewidths eases reference regulation in multi-referenceamplifiers considerably.

Although the present invention has been described with reference to thedisclosed embodiments, persons skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. Such modifications are well within the skillof those ordinarily skilled in the art. Accordingly, the invention isnot limited except as by the appended claims.

1. (canceled)
 2. A switching amplifier, comprising: a circuit configuredto receive a first voltage and generate a second voltage based, at leastin part, on the first voltage; a controller configured to generate firstand second control signals; a first switching circuit configured togenerate first pulsewidths based on the first voltage in accordance withthe first control signal; and a second switching circuit configured togenerate second pulsewidths based on the second voltage in accordancewith the second control signal; wherein the controller is furtherconfigured to generate the second pulsewidths to prevent the circuitfrom sourcing current.
 3. The switching amplifier of claim 1, whereinthe circuit comprises a voltage regulator.
 4. The switching amplifier ofclaim 1, wherein the controller is coupled to the first switchingcircuit, and wherein the controller is further configured to provide thefirst control signal to the first switching circuit.
 5. The switchingamplifier of claim 1, wherein the first switching circuit is furtherconfigured to provide the first pulsewidths to a load, and wherein thesecond switching circuit is further configured to provide the secondpulsewidths to the load.
 6. The switching amplifier of claim 1, whereinthe first switching circuit comprises a plurality of switching devices.7. The switching amplifier of claim 6, wherein the second switchingcircuit comprises another plurality of switching devices.
 8. Theswitching amplifier of claim 1, wherein the first switching circuit isfurther configured to provide coarse control of power to a load, andwherein the second switching circuit is further configured to providefine control of power to the load.
 9. The switching amplifier of claim1, wherein the controller is further configured to suppress minimumpulsewidths of the second voltage on one side of a load.
 10. A switchingamplifier, comprising: a first switching circuit configured to provide afirst voltage to a load; a second switching circuit configured toprovide a second voltage to the load, wherein the second voltage isgenerated by a circuit based, at least in part, on the first voltage;and a modulator configured to provide control signals to the first andsecond switching circuits such that the first switching circuit isfurther configured to provide a coarse control of power to the load andthe second switching circuit is further configured to provide a finecontrol of power to the load, wherein the modulator is furtherconfigured to control the second switching circuit and thereby preventthe circuit from providing current flow in two directions.
 11. Theswitching amplifier of claim 10, wherein the circuit is a regulatorcircuit.
 12. The switching amplifier of claim 10, wherein the firstswitching circuit comprises a first plurality of transistors and thesecond switching circuit comprises a second plurality of transistors.13. The switching amplifier of claim 10, wherein the modulator isconfigured to prevent the second switching circuit from providing thesecond voltage to the load when providing the second voltage to the loadwould cause the circuit to provide current flow in two directions. 14.The switching amplifier of claim 10, wherein the modulator is configuredto suppress minimum pulsewidths of the second voltage on one side of theload.
 15. The switching amplifier of claim 10, wherein the secondvoltage is lower than the first voltage.
 16. A method of applying asignal to a load, the method comprising: receiving, at a circuit, afirst voltage; generating a second voltage based, at least in part, onthe received first voltage; coupling a first side of the load to thefirst voltage in accordance with a first control signal to providecoarse control of power to the load; coupling a second side of the loadto the second voltage in accordance with a second control signal toprovide fine control of power to the load, wherein the second controlsignal further prevents said coupling a second side of the load to thesecond voltage when coupling the second side of the load to the secondvoltage would cause the circuit to source current.
 17. The method ofclaim 16, wherein said coupling a first side of the load to the firstvoltage comprises coupling the first side of the load to the firstvoltage using a first switching circuit.
 18. The method of claim 17,wherein said coupling a second side of the load to the second voltagecomprises coupling the second side of the load to the second voltageusing a second switching circuit.
 19. The method of claim 16, whereinthe circuit comprises a regulator.
 20. The method of claim 16, whereinthe first and second control signals comprise pulsewidth-modulatedsignals.
 21. The method of claim 16, wherein the second control signalfurther prevents minimum pulsewidths of the second voltage from beingapplied to the load when minimum pulsewidths of the second voltage wouldcause the circuit to source current.